Pharmaceutical composition for the treatment of pain

ABSTRACT

The present invention relates to a pharmaceutical composition comprising a combination of trazodone or a salt thereof, and gabapentin or a salt or prodrug thereof, said combination having a synergistic effect in the treatment of pain.

FIELD OF THE INVENTION

The present invention relates to a pharmaceutical composition comprisinga combination of trazodone and gabapentin, said combination having asynergistic effect in the treatment of pain, in particular chronic pain.

BACKGROUND OF THE INVENTION

Physiological pain is an important protective mechanism designed to warnof danger from potentially injurious stimuli from the externalenvironment.

Intense acute pain and chronic pain may involve the same pathways drivenby pathophysiological processes and as such cease to provide aprotective mechanism and instead contribute to debilitating symptomsassociated with a wide range of disease states. Pain is a feature ofmany trauma and disease states. When a substantial injury, via diseaseor trauma, to body tissue occurs the characteristics of nociceptoractivation are altered.

There are a number of typical pain subtypes: 1) spontaneous pain whichmay be dull, burning, or stabbing; 2) pain responses to noxious stimuliare exaggerated (hyperalgesia); 3) pain is produced by normallyinnocuous stimuli (allodynia).

Therefore pain can be divided into a number of different areas becauseof differing pathophysiology, these include nociceptive pain,inflammatory pain, chronic pain, neuropathic pain, and so on.

Nociceptive pain is induced by tissue injury or by intense stimuli withthe potential to cause injury.

Neuropathic pain is defined as pain initiated or caused by a primarylesion or dysfunction in the nervous system.

The inflammatory process is a complex series of biochemical and cellularevents activated in response to tissue injury or the presence of foreignsubstances, which result in swelling and pain. Arthritic pain makes upthe majority of the inflammatory pain population. Rheumatoid disease isone of the commonest chronic inflammatory conditions in developedcountries and rheumatoid arthritis is a common cause of disability.Other types of inflammatory pain include but are not limited toinflammatory bowel diseases (IBD).

Chronic pain represents a series of pathologies from which, on average,about 10-20% of the adult population suffers. Chronic pain is generallyassociated with clinical conditions characterized by chronic and/ordegenerative lesions.

Chronic pain differs from acute pain mainly by the duration. Acute painhas a duration of a few days or weeks, correlated with recovery from theevent that caused the pain (trauma, burns, intense efforts, surgical ordental interventions, and the like). On the other hand, chronic painpersists for months and even years, causing muscular tension, limitedmobility, fatigue, loss of appetite and apathy.

Typical examples of pathologies characterized by chronic pain arerheumatoid arthritis, osteoarthritis, fibromyalgia, neuropathies, etc.[Ashbum Mass., Staats PS. Management of chronic pain. Lancet 1999; 353:1865-69].

Chronic pain, and in particular neuropathic pain, is often debilitatingand is a cause of loss of working capacity and of poor quality of life.Economic and social damage thus also follow.

The analgesic drugs currently used in the treatment of neuropathic paininclude non-steroidal anti-inflammatory drugs (NSAID), anti-depressants,opioid analgesics, and anticonvulsants [Woolf C J, Mannion R J,Neuropathic pain: aetiology, symptoms, mechanism, and management. Lancet1999; 353: 1959-1964].

However, chronic pain, and in particular neuropathic pain, isnotoriously difficult to treat with the drugs currently available.Consequently, the development of novel drugs has always been one of themain objectives of the pharmaceutical industry.

EP1663398B1 discloses a combination of an alpha-2-delta ligand such asgabapentin and pregabalin with a selective noradrenaline re-uptakeinhibitor such as (S,S)-reboxetine for the treatment of neuropathicpain.

WO 2013/002584 discloses a pain suppressive composition comprising twoor more components selected from (a) 5-HT2 receptor antagonist, (b) aP2X receptor antagonist, and (c) any one of glycine receptor agonist, aglycine transporter antagonist, a gamma-aminobutyric acid (GABA)receptor agonist, and a GABA transporter 1 (GABA1) antagonist.

A review of the use of gabapentin in the treatment of neuropathic painhas been published by Kukkar A. et al, “Implications and mechanism ofaction of gabapentin in neuropathic pain”, Arch. Pharm. Res. (2013)36:237-251. A superior effect was observed by the combination ofgabapentin and venlafaxine in the rat spared nerve injury (SNI) model ofneuropathic pain and by co-administration of gabapentin with donepeziland/or duloxetine against spinal nerve ligation-induced neuropathic painand in spared nerve injury model.

Trazodone and a combination of trazodone and pregabalin have beenstudied in a 12- and 24-week treatment of fibromyalgia as disclosed inMorillas-Arques et al., “Trazodone for the treatment of fibromyalgia: anopen-label, 12-week study”, BMC Musculoskeletal Disorders 2010, 11:204and Calandre et al., “Trazodone plus pregabalin combination in thetreatment of fibromyalgia: a two-phase, 24-week, open-label uncontrolledstudy”, BMC Musculoskeletal Disorders 2011, 12:95.

A combination of trazodone with fentanyl and paracetamol has beenstudied in a mouse model of visceral pain, showing a potent synergisticantinociceptive effect, as disclosed in Fernández-Dueñas et al.,“Fentanyl-trazodone-paracetamol triple drug combination: Multimodalanalgesia in a mouse model of visceral pain”, Pharmacology, Biochemistryand Behavior 98 (2011) 331-336.

Despite the numerous research efforts directed towards identifying asuitable analgesic compound, there is a significant number of patientswhose pain condition still lacks a suitable treatment [Scholz J, Woolf CJ. Can we conquer pain? Nat Neusci. 2002; 5: 1062-76] and there is acontinuous need to find novel treatments able to improve the patientconditions.

SUMMARY OF THE INVENTION

The Applicant faced the problem to provide a pharmaceutical compositionfor the treatment of pain, in particular chronic pain, having animproved activity when compared to the compositions known in the art.

The Applicant has found that trazodone can be used in association withgabapentin.

In particular, the Applicant has now surprisingly found that thecombination of trazodone with gabapentin has a synergistic effect inreducing pain in two experimental models in rats represented by (i) theinflammatory pain induced by injection of acetic acid, and (ii) thechronic pain induced by ligature of the sciatic nerve.

As is known to those skilled in the art, the abovementioned experimentalmodels may be considered as predictive of activity in man.

The experimental model of injection of acetic acid in rats represents anevaluation of the antinociceptive activity of the combination of theinvention in a model of inflammatory pain.

Typical examples of human pathologies characterized by the dysfunctionsdescribed in the abovementioned experimental model and characterized bythe presence of inflammatory pain are oedema, erythema, articularinflammation, osteoarthritis, rheumatoid arthritis and arthrosis.

The experimental model of ligature of the sciatic nerve in ratsrepresents a neuropathy that reproduces a series of responses similar tothose observed in man in numerous traumatic and pathological conditionsassociated with neuropathic pain. The reason for this is that ligatureof the sciatic nerve is capable of inducing a syndrome associated withthe activation of specific circuits dedicated to controlling theperception of pain and characterized by the appearance of allodynia,hyperalgesia and spontaneous pain. This model is well known toconstitute a valid instrument for studying drugs to be used in thetreatment of neuropathic pain in man, and in particular in controllingconditions such as allodynia and hyperalgesia.

Typical examples of human pathologies characterized by the dysfunctionsdescribed in the abovementioned experimental model and characterized bythe presence of neuropathic pain are diabetes, cancer, immunodeficiency,trauma, ischemia, multiple sclerosis, sciatica, trigeminal neuralgia,fibromyalgia and post-herpetic syndrome.

Thus, in a first aspect, the present invention relates to apharmaceutical composition comprising a synergistic combination oftrazodone or a salt thereof, and gabapentin or a salt or prodrugthereof, and at least one pharmaceutically acceptable excipient, for usein the treatment of pain.

A second aspect of the present invention relates to a method for thetreatment of pain, comprising administering to a subject in need thereofa synergistically effective amount of trazodone or a salt thereof andgabapentin or a salt or prodrug thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 visually shows the results of example 1. The ED₅₀ values of eachdrug (trazodone and gabapentin) are plotted on the X and Y axis. Theline connecting both points is the theoretical line of additivity. Theexperimental ED₅₀ value of the trazodone and gabapentin combination(ED₅₀mix) is found significantly below the theoretical additive ED₅₀value (ED₅₀add).

FIG. 2 visually shows the results of example 2. The number of writhingcounted within the 10 minutes after the administration of acetic acidare reported on the Y axis for each group of mice administered with thevehicle or ineffective amounts of trazodone and gabapentin alone or incombination as reported on the X axis.

FIG. 3 visually shows the results of example 3. In FIG. 3 a , theinhibition percentage, expressed with reference to the result obtainedwith the vehicle, is reported on the Y axis for each group of miceadministered with trazodone and gabapentin alone or in combination witha weight ratio as reported on the X axis. FIG. 3 b only reported on theY axis the inhibition percentage of the trazodone/gabapentin combinationwith a weight ratio as reported on the X axis.

FIG. 4 visually shows the results of example 4. The reversal percentage,expressed with reference to the result obtained with the vehicle, isreported on the Y axis for each group of rats administered withtrazodone and gabapentin alone or in combination as reported on the Xaxis.

DETAILED DESCRIPTION OF THE INVENTION

In the present description, the expression “synergistic effect” meansthat the combination of trazodone with gabapentin inhibits the painresponse in mice or rats at a concentration lower than the sum of theconcentrations of trazodone and of gabapentin used alone for the samepain test to obtain the same inhibition. In other words, a synergisticeffect means that the combination of trazodone with gabapentin iseffective in producing more than the additive effect of each componentin the same pain test.

The pharmaceutical composition according to the present inventioncomprises trazodone or a salt thereof in an amount to provide anineffective dosage if taken alone.

Preferably, the pharmaceutical composition according to the presentinvention contains trazodone or a salt thereof in an amount to provide adosage equal to or lower than 1 mg/kg, more preferably equal to or lowerthan 0.5 mg/kg, and even more preferably equal to or lower than 0.2mg/kg.

Preferably, the pharmaceutical composition according to the presentinvention contains trazodone or a salt thereof in an amount to provide adosage equal to or higher than 0.01 mg/kg, more preferably equal to orhigher than 0.05 mg/kg, and even more preferably equal to or higher than0.1 mg/kg.

Accordingly, the pharmaceutical composition according to the presentinvention may contain trazodone or a salt thereof in any amount able toprovide a dosage ranging from 0.01 mg/kg to 1 mg/kg, preferably rangingfrom 0.05 mg/kg to 0.5 mg/kg, more preferably from 0.1 mg/kg to 0.2mg/kg, such as, for example, 0.02, 0.04, 0.07, 0.11, 0.12, 0.13, 0.14,0.15, 0.16, 0.17, 0.18, 0.19, 0.2, 0.25, 0.3, 0.4, 0.6, 0.7, 0.8, and0.9 mg/kg, and the like.

Advantageously, the pharmaceutical composition according to the presentinvention contains trazodone or a salt thereof in an amount to provide adosage equal to about 0.15 mg/kg.

Accordingly, the pharmaceutical composition according to the presentinvention may contain trazodone or a salt thereof in an amount equal toor lower than 70 mg, more preferably equal to or lower than 35 mg, andeven more preferably equal to or lower than 15 mg.

The pharmaceutical composition according to the present inventioncomprises gabapentin or a salt or prodrug thereof in an amount toprovide an ineffective dosage if taken alone.

Preferably, the pharmaceutical composition according to the presentinvention contains gabapentin or a salt or prodrug thereof in an amountto provide a dosage equal to or lower than 15 mg/kg, more preferablyequal to or lower than 5 mg/kg, and even more preferably equal to orlower than 2 mg/kg.

Preferably, the pharmaceutical composition according to the presentinvention contains gabapentin or a salt or prodrug thereof in an amountto provide a dosage equal to or higher than 0.1 mg/kg, more preferablyequal to or higher than 0.5 mg/kg, and even more preferably equal to orhigher than 1 mg/kg.

Accordingly, the pharmaceutical composition according to the presentinvention may contain gabapentin or a salt or prodrug thereof in anyamount able to provide a dosage ranging from 0.1 mg/kg to 15 mg/kg,preferably ranging from 0.5 mg/kg to 5 mg/kg, more preferably from 1mg/kg to 2 mg/kg, such as, for example, 0.2, 0.4, 0.7, 1.1, 1.2, 1.3,1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.5, 3.0, 4.0, 6.0, 7.0, 8.0, and 9.0mg/kg, and the like.

Advantageously, the pharmaceutical composition according to the presentinvention contains gabapentin or a salt or prodrug thereof in an amountto provide a dosage equal to about 1.5 mg/kg.

Accordingly, the pharmaceutical composition according to the presentinvention may contain gabapentin or a salt or prodrug thereof in anamount equal to or lower than 1000 mg, more preferably equal to or lowerthan 350 mg, and even more preferably equal to or lower than 150 mg.

Advantageously, the pharmaceutical composition according to the presentinvention contains an amount of trazodone or a salt thereof and anamount of gabapentin or a salt or prodrug thereof such as to provide atrazodone to gabapentin weight ratio ranging from 1:40 to 10:1,preferably from 1:20 to 1:1, and more preferably from 1:15 to 1:5.

Any value within the above mentioned trazodone to gabapentin weightratio can be used, such as, for example, 1:40, 1:39, 1:38, 1:37, 1:36,1:35, 1:34, 1:33, 1:32, 1:31, 1:30, 1:29, 1:28, 1:27, 1:26, 1:25, 1:24,1:23, 1:22, 1:21, 1:20, 1:19, 1:18, 1:17, 1:16, 1:15, 1:14, 1:13, 1:12,1:11, 1:10, 1:9, 1:8, 1:7, 1:6, 1:5, 1:4, 1:3, 1:2, 1:1, 2:1, 3:1, 4:1,5:1, 6:1, 7:1, 8:1, and 9:1.

Advantageously, the pharmaceutical composition according to the presentinvention contains an amount of trazodone or a salt thereof and anamount of gabapentin or a salt or prodrug thereof such as to provide atrazodone to gabapentin weight ratio of about 1:10.

Prodrugs of gabapentin are included in the scope of the instantinvention. The chemically modified drug, or prodrug, should have adifferent pharmacokinetic profile to the parent, enabling easierabsorption across the mucosal epithelium, better salt formulation and/orsolubility, improved systemic stability (for an increase in plasmahalf-life, for example).

These chemical modifications may be (i) ester derivatives (ii) amidederivatives, (iii) carbamate derivatives, (iv) N-acyloxyalkylderivatives, (v) N-acyloxyalkoxycarbonyl derivatives, (vi) peptides, and(vii) any combination thereof.

The ester can be derived from the carboxylic acid moiety of the drugmolecule by known means. The amide may be derived from the carboxylicacid moiety or the amine moiety of the drug molecule by known means.Ester or amide derivatives may be cleaved by, for example, esterases orlipases.

The peptide may be coupled to the drug molecule via amide bond formationwith the amine or carboxylic acid moiety of the drug molecule by knownmeans. Peptides may be recognized by specific or nonspecificproteinases.

The carbamate, N-acyloxyalkyl and N-acyloxyalkoxycarbonyl can be derivedfrom the amine moiety of the drug molecule by known means. Suchderivatives can be cleaved by esterases and/or decomposes spontaneously.

Useful examples of suitable prodrugs are described, for example, inStella V. J. et al, “Prodrug strategies to overcome poor watersolubility”, Advance Drug Delivery Reviews 59 (2007) 677-694, and inSimplicio A. L. et al., “Prodrugs for Amines”, Molecules 2008, 13,519-547.

An advantageously useful example of gabapentin prodrug is the compound(1-{[({(1RS)-1-[isobutyryloxy]ethoxy}carbonyl)amino] methyl} cyclohexyl)acetic acid, commonly known as gabapentin enacarbil.

Salts of trazodone and gabapentin are included in the scope of theinstant invention. The salt can be formed with physiologicallyacceptable organic and inorganic compounds having an acid or a basicfunction.

Typical examples of suitable physiologically acceptable inorganic acidsare hydrochloric acid, hydrobromic acid, sulfuric acid, phosphoric acidand nitric acid.

Typical examples of suitable physiologically acceptable organic acidsare acetic acid, ascorbic acid, benzoic acid, citric acid, fumaric acid,lactic acid, maleic acid, methanesulfonic acid, oxalic acid,paratoluenesulfonic acid, benzenesulfonic acid, succinic acid, tannicacid and tartaric acid.

Typical examples of suitable physiologically acceptable inorganic basesare hydroxides, carbonates and hydrogen carbonates of ammonium, calcium,magnesium, sodium and potassium, for instance ammonium hydroxide,calcium hydroxide, magnesium carbonate, sodium hydrogen carbonate andpotassium hydrogen carbonate.

Typical examples of suitable physiologically acceptable organic basesare: arginine, betaine, caffeine, choline, N,N-dibenzylethylenediamine,diethylamine, 2-diethylam inoethanol, 2-dimethylaminoethanol,etha-nolamine, ethylenediamine, N-ethylmorpholine, N-ethylpiperidine,N-methylglucamine, glucamine, glucosamine, histidine,N-(2-hydroxyethyl)piperidine, N-(2-hydroxyethyl)pyrrolidine,isopropylamine, lysine, methylglucamine, morpholine, piperazine,piperidine, theobromine, triethylamine, trimethylamine, tripropylamineand tromethamine.

Preferably, the pharmaceutical composition according to the presentinvention is for systemic use.

More preferably the pharmaceutical composition according to the presentinvention is formulated for oral or parenteral administration.

Preferably, the pharmaceutical composition according to the presentinvention is prepared in suitable dosage forms.

Examples of suitable dosage forms are tablets, capsules, coated tablets,granules, and solutions and syrups for oral administration; creams,ointments transdermal patch and antiseptic plasters for topicaladministration; suppositories for rectal administration and sterilesolutions for administration by injection, or aerosol or ophthalmicadministration.

The dosage forms of the pharmaceutical composition of the presentinvention may be prepared according to techniques that are well known topharmaceutical chemists, including mixing, granulation, tableting,dissolution, sterilization and the like.

Advantageously, these dosage forms are formulated so as to ensure acontrolled release over time of a compound of the abovementioned generalformula (I) or of a pharmaceutically acceptable salt thereof.Specifically, depending on the type of therapy, the required releasetime may be very short, normal or long.

The pharmaceutically acceptable excipient can be selected from the groupcomprising thickeners, glidants, binders, disintegrants, fillers,diluents, preservative, stabilizers, surfactants, buffers, fluidizers,lubricants, humectants, absorbents, salts for regulating osmoticpressure, emulsifiers, flavorings, colorants, sweeteners, and the like.

The pharmaceutical composition according to the present invention iseffective for the treatment of pain.

The Applicant has found that the combination of trazodone withgabapentin has a synergistic effect in reducing pain in two experimentalmodels in animals represented by (i) the inflammatory pain induced inmice by injection of acetic acid, and (ii) the chronic pain induced inrats by ligature of the sciatic nerve.

Accordingly, the pharmaceutical composition according to the presentinvention can be used to treat effectively and with high safety allkinds of chronic pain, either inflammatory or neuropathic in origin.

Preferred examples of chronic pain treated according to the presentinvention are the following:

inflammatory pain induced by oedema, erythema, articular inflammation,osteoarthritis, rheumatoid arthritis and arthrosis; and

neuropathic pain induced by diabetes, cancer, immunodeficiency, trauma,ischemic, multiple sclerosis, sciatica, trigeminal neuralgia,fibromyalgia and post-herpetic syndrome.

The examples that follow are intended for further illustration of thepresent invention, though without limiting it.

EXAMPLES Description of the Test Methods Writhing Test in Mice

The test compounds were evaluated in the model of acetic acid-inducedwrithing in mice (Stock J. L. et al., J. Clin. Inv. 2001, 107: 325 331).

Writhing test is a chemical method used to induce pain of peripheralorigin by injection of acetic acid in mice. Analgesic activity isinferred from decrease in the frequency of writhing.

This test allowed evaluation of the antinociceptive activity of thecompounds of the invention in a model of inflammatory pain.

Writhing is defined as a stretch, tension to one side, extension of hindlegs, contraction of the abdomen so that the abdomen of mice touches thefloor, turning the trunk (twist).

Male CD-1 mice weighing 20-25 g were used for the test.

The mice were orally treated with trazodone and gabapentin alone and incombination, or vehicle such as distilled water.

One hour after the treatment, an intraperitoneal injection of aceticacid (0.7% v/v in physiological saline, 16 μl/g of body weight) wasgiven to the mice to induce inflammatory pain and to check the effectsof the test compound on the nociceptive response.

Five minutes after the administration of acetic acid and for thefollowing 10 minutes the number of writhes was measured, whichrepresents the parameter for evaluation of the nociceptive response. Theanalgesic activity is evaluated as % inhibition.

Ligature of the Sciatic Nerve in Rats

Male CD rats weighing 200-250 g on arrival were used.

The allodynia was induced by ligature under anaesthesia of the sciaticnerve of the left hind leg [Seltzer Z, Dubner R, Shir Y. A novelbehavioural model of neuropathic pain disorders produced in rats bypartial sciatic nerve injury. Pain 1990; 43: 205-218; Bennet G J, Xie YK. A peripheral mononeuropathy in rat that produces disorders of painsensation like those seen in man. Pain 1988; 33: 87-107].

Two weeks after ligature of the sciatic nerve, rats who showed areduction of at least 50% in the response threshold recorded before theintervention were selected. The pain threshold was measured with a vonFrey machine, which makes it possible, by applying a gradual increase inpressure to the paw of the left hind leg of the rat, to record thenociceptive response, expressed in grams, corresponding to the moment atwhich the animal withdraws the leg.

Immediately after a pre-treatment measurement, the rats were treatedwith trazodone and gabapentin alone and in combination, or vehicle suchas distilled water. After-treatment measurement was made at one hourfollowing administration. The analgesic activity is evaluated as %reversal.

Results Example 1

The ED50 values of trazodone, gabapentin and trazodone/gabapentincombination with a trazodone/gabapentin weight ratio equal to 1/10 hasbeen determined using the writhing test in mice.

The results are summarized in the following Table 1.

TABLE 1 Drug(s) ED50 (mg/kg) Trazodone 1.2 Gabapentin 27.8Trazodone/Gabapentin 2.5

As illustrated in the FIG. 1 , the theoretical additive ED50 of thetrazodone/gabapentin combination should have been 14.5 mg/kg, soresulting in an interaction index of about 0.17, providing a strongindication of synergistic interaction.

Example 2

The writhing test in mice was repeated by administering to four groupsof mice ineffective increasing amounts of trazodone and gabapentinalone, or in combination with a trazodone/gabapentin weight ratio equalto 1/10, according to the following Table 2, and vehicle alone.

The results of each test, expressed as reduction percentage of writhingnumber compared with the vehicle, are summarized in Table 2 andillustrated in FIGS. 2 a, 2 b and 2 c .

TABLE 2 Writhing number Drug(s) Amount (mg/kg) reduction % TEST 1 - FIG.2a Trazodone 0.15 16.0 Gabapentin 1.5 27.1 Trazodone/Gabapentin0.15/1.5  57.8 TEST 2 - FIG. 2b Trazodone 0.5 44.4 Gabapentin 5 38.1Trazodone/Gabapentin 0.5/5   48.1 TEST 3 - FIG. 2c Trazodone 1.5 57.1Gabapentin 15 45.9 Trazodone/Gabapentin 1.5/1.5 66.4

The results surprisingly demonstrated that the synergistic effect of thecombination of trazodone gabapentin were evident only at the lowestdosage of trazodone and gabapentin employed in test 1.

Example 3

The writhing test in mice was repeated by administering to four groupsof mice different ineffective amounts of trazodone and gabapentin alone,or in combination with different trazodone/gabapentin weight ratio,according to the following Table 3, and vehicle alone.

The results of each test, expressed as reduction percentage of writhingnumber compared with the vehicle, are summarized in Table 3 andillustrated in FIGS. 3 a and 3 b .

TABLE 3 Writhing number Drug(s) Amount (mg/kg) reduction % TEST 1 -RATIO 1:20 Trazodone  0.075 2.1 Gabapentin 1.5  29.2Trazodone/Gabapentin 0.075/1.5  47.6 TEST 2 - RATIO 1:10 Trazodone 0.1516.0 Gabapentin 1.5  27.1 Trazodone/Gabapentin 0.15/1.5  57.8 TEST 3 -RATIO 1:5 Trazodone 0.15 16.3 Gabapentin 0.75 22.4 Trazodone/Gabapentin0.15/0.75 44.4 TEST 4 - RATIO 1:1 Trazodone 0.15 12.2 Gabapentin 0.1528.3 Trazodone/Gabapentin 0.15/0.15 30.7 TEST 5 - RATIO 5:1 Trazodone0.75 57.4 Gabapentin 0.15 30.2 Trazodone/Gabapentin 0.75/0.15 58.5 TEST6 - RATIO 10:1 Trazodone 1.5  59.7 Gabapentin 0.15 27.2Trazodone/Gabapentin  1.5/0.15 67.9

The results surprisingly demonstrated that the combination of trazodoneand gabapentin having a trazodone/gabapentin weight ratio equal to 1:10showed the most enhanced synergistic analgesic activity.

Example 4

The test of ligature of the sciatic nerve in rats was employed toconfirm the synergic effect of the combination of trazodone andgabapentin in the treatment of chronic neuropathic pain.

The test was done by administering to five groups of rats ineffectiveamounts of trazodone and gabapentin alone, or in combination with atrazodone/gabapentin weight ratio equal to 1:10, and an effective amountof gabapentin according to the following Table 4 and vehicle alone.

The results of the test, expressed as reversal percentage compared withthe vehicle, are summarized in Table 4 and illustrated in FIG. 4 .

TABLE 4 Drug(s) Amount (mg/kg) Reversal % Trazodone 0.3 3.1 Gabapentin3.0 3.7 Trazodone/Gabapentin 0.3/3.0 44.3 Gabapentin 100 37.0

The results confirmed the synergic effect of the combination oftrazodone and gabapentin in the treatment of chronic neuropathic pain,which resulted in an activity even higher than the activity obtainedwith a conventional effective dose of gabapentin alone.

The invention claimed is:
 1. A method of treating pain, comprisingadministering to a subject in need thereof trazodone or a salt thereofand gabapentin or a salt or prodrug thereof as the only active agents ina combined effective synergistic amount, wherein said pain is selectedfrom the group consisting of chronic pain, inflammatory pain, andneuropathic pain, wherein said trazodone or a salt thereof isadministered in an amount equal to or lower than 0.2 mg/kg and saidgabapentin or a salt or prodrug thereof is administered in an amountequal to or lower than 2 mg/kg, and wherein the trazodone or saltthereof and gabapentin or a salt or prodrug thereof weight ratio is from1:15 to 1:5.
 2. The method according to claim 1, wherein said pain ischronic pain.
 3. The method according to claim 1, wherein said pain isinflammatory pain induced by oedema, erythema, articular inflammation,osteoarthritis, rheumatoid arthritis, or arthrosis.
 4. The methodaccording to claim 1, wherein said pain is neuropathic pain.
 5. Themethod according to claim 1, wherein said pain is neuropathic paininduced by diabetes, cancer, immunodeficiency, trauma, ischemia,multiple sclerosis, sciatica, trigeminal neuralgia, fibromyalgia, orpost-herpetic syndrome.
 6. The method according to claim 1, wherein saidpain is neuropathic pain induced by diabetes.
 7. The method according toclaim 1, which comprises administering trazodone or a salt thereof in anamount equal to about 0.15 mg/kg and gabapentin or a salt or prodrugthereof in an amount equal to about 1.5 mg/kg.
 8. The method accordingto claim 1, which comprises administering trazodone or a salt thereofand gabapentin or a salt or prodrug thereof in a trazodone to gabapentinweight ratio of about 1:10.